Selenium (Se) is a trace element that is essential in the diet of horses.

Se deficiency is a serious problem in many parts of North America, mainly due to soil deficiency, which results in low Se concentrations in feedstuffs grown in these soils. Foals are especially susceptible to Se deficiency, which can be seen clinically as white muscle disease and also is associated with decreased immune function in many species. In foals, septicemia (blood infection) as a result of inadequate immunity is common and often fatal. There is little information available on the effects of maternal supplementation on the Se status and immune function of neonatal foals.
In previous work funded by the SJDAWC7, it was shown that many mares have deficient levels of Se, resulting in Se deficiency in their foals. The objective of the current project was to investigate the effect of Se supplementation and the source of Se on the Se status of pregnant mares and the Se status and immune function of their foals. The ultimate goal is to correct Se deficiency in mares and foals by the most effective and practical means, and thereby improve preventive health care.
Twenty pregnant Standardbred mares were randomly assigned to two groups, one of which received inorganic and the other organic  selenium supplementation for two months prior to, and one month after, foaling. There were no significant differences in maternal plasma or red blood cell Se concentrations between the organic and inorganic groups at any time during the study, and the same was true in the foals at birth. At one month of age, there was a significant between-group difference in red blood cell Se concentration of foals (p < 0.05), with
higher levels in the organic group. Selenium source did not affect colostrum or milk Se concentration; however, colostrum and milk Se concentrations were closely correlated with the plasma and red blood cell Se concentrations in the mare. This finding may be of future use to predict selenium concentrations in colostrum and milk based on the Se status of the mare in late pregnancy. The effect of Se on immune
function was also examined—Se source influenced relative gene expression of some lymphocytic cytokines. No Se source appeared to be consistently superior to the other and the significance of these observations remains to be clarified.
To conclude, Se is an essential micronutrient of horses, with important effects on antioxidant status and immune response. This study is one of a series completed over the past five years that have contributed to an understanding of how different Se sources influence the Se status of mares and their foals, as well as insights as to how the source of Se may influence the immune function of foals8, 9, 10. The ultimate goal is to present owners and advisors with a range of options, including pros and cons, for ensuring selenium adequacy that can fit all management situations.

(J Wichtel, F Markham, J McClure, M McNiven, M Wichtel, J Montgomery)